9. Select the type and parameters of the RCD
9.3. Nominal load current I n
Nominal load current I n is selected from the group of 6 (10), 16, 25, 40, 63, 80, 100, 125 A. For RCD value of this current is determined typically section conductors in the device design and power contacts. Since the RCD must be protected consistent protection device (ROM), the rated load current RCD should be coordinated with the nominal current ROM.
As mentioned in Sec. 7.3 of these Recommendations, the nominal load current RCD should be equal to or one rank higher than the nominal current of the protective device serial. In foreign regulations (for example, in the Austrian OVE EN1, T1, §12.12) there is a requirement to increase the level of the nominal load current relative to the rated RCD current serial protective device. This means that, for example, in a circuit breaker protected with a rated current of 25 A, determined by the method described in Sec. 7, must be installed an RCD with a rated current of 40 A (Table. 9.1).
Table 9.1.
| Device | Rated load current | ||||||
| ROM | 10 | 16 | 25 | 40 | 63 | 80 | 100 |
| RCD | 16 | 25 | 40 | 63 | 80 | 100 | 125 |
The feasibility of such a requirement can be explained by a simple example.
If the RCD and the circuit breaker have the same nominal current, then the current flows exceeding the nominal, for example, by 45%, ie overload current, this current will be off the circuit breaker for up to one hour. This means that the time period RCD will be overloaded.
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